A known reclining device for a vehicle for adjusting an angle of a seatback is disclosed in JP2007-261325A (which will be referred to as reference 1 hereinbelow). The reclining device according to reference 1 is provided between a lower frame, which serves as a supporting member of a seat cushion and is provided at each left and right ends of a seat and an upper frame, which serves as a supporting member of the seatback. The reclining device includes a planetary-gear power transmission mechanism. In the power transmission mechanism, an external gear, which is formed at a lower bracket fixed at the lower frame, and an internal gear, which is formed at an upper bracket fixed at the upper frame, are engaged with each other. Wedge-shaped wedge members are disposed between the lower bracket and the upper bracket. When a driving shaft is rotated, the wedge members are rotated in response to the rotation of the driving shaft, an axis of the external gear revolves around an axis of the internal gear and an engagement position between the external gear and the internal gear is rotated in a planar surface of the planetary rotation. When the engagement position between the external gear and the internal gear is rotated by one rotation, the upper bracket is rotated relative to the lower bracket by an angle corresponding to a difference in number of teeth between the external and internal gears. The wedge members are provided to be in pairs. The wedge members are applied with a biasing force by a ring-shaped spring, serving as a biasing member, so that the external gear is thrust to the internal gear. Consequently, a backlash is prevented from being generated at the engagement position between the external gear and the internal gear. Therefore, an angular position of the seatback is firmly held. Further, the angular position of the seatback may be changed by forward and reverse rotations of the driving shaft by means of a motor having a speed reduction mechanism.
According to the seat driving apparatus assembled with the gear transmission mechanism, the backlash is prevented from being generated at the gear transmission mechanism and the angular position of the seatback is firmly held by an engagement of the wedge members. In order to further adjust the angle of the seatback, the engagement of the wedge members needs to be released. Therefore, the wedge members are applied with a load to be displaced by a driving force of the actuator. However, an operation by means of preparatory driving at low load (which will be referred to as a low load preparatory operation, hereinbelow) is provided before the wedge members are applied with the load when the seatback is operated in an opposite direction from the previous operation. Consequently, the low load preparatory operation and a subsequent impact of load application are transmitted to the seatback as uncomfortable shaking.
A need thus exits for a seat apparatus for a seat driving apparatus, which is not susceptible to the drawback mentioned above.